1. Field of the Invention
This invention relates to a load detector which can detect loads with high accuracy insofar as it is important for the attainment of desired control of a machine to detect loads which each of the mechanical parts making up the machine receives.
2. Discussion of the Background
As to load detectors which have conventionally been used for the detection of such loads, reference may be made to U.S. Pat. Nos. 3,695,096 issued Oct. 3, 1972 to Ali Umit Kutsay, 4,165,637 issued Aug. 28, 1979 to William H. Kooman, and 4,199,978 issued Apr. 29, 1980 to Folke von Knorring et al. U.S. Pat. No. 3,695,096 discloses a strain detecting load cell formed of a coupling member such as pin or bolt, which member defines a hole along its central axis, and strain gauges bonded to the circumferential wall of the hole, to thereby detect a shear force corresponding to each load to be exerted to the coupling member. The strain detecting load cell of the above structure has poor detection sensitivity and is thus unable to detect with desired accuracy each load to be applied to the coupling member, because the wall thickness of the coupling member on which the strain gauges are bonded must be large in order to keep its strength at a sufficient level.
U.S. Pat. No. 4,165,637 discloses a force-transferring device in which two holes are formed along the central axis of a pin in such a way that a plane containing the central axes of these holes extends perpendicularly to the direction of action of each force, and strain gauges are provided on the inner walls of these holes so as to detect a shear force corresponding to each load to be applied to the pin. The above force-transferring device is accompanied by a potential danger that the strength and rigidity of the pin would be reduced due to the formation of two holes therethrough and may be broken or may develop undesirous deformations when applied with a large load, although it seems to be able to detect with desired accuracy loads to be applied to the pin as the pin has small cross-sectional areas at areas where the pins are respectively provided.
The inventions of U.S. Pat. Nos. 3,695,096 and 4,165,637 were both completed, fundamentally speaking, on the basis of the same technical concept. Namely, these inventions are able to enlarge shear strains (in other words, to improve the sensitivity) at pin portions, in which shear deformations occur, only by making the pin portions weaker. It is however required for almost all of ordinary applications to impart sufficient degrees of strength and rigidity to each pin and at the same time to improve its sensitivity as a sensor while maintaining the dimensions of the pin at conventional levels. Neither U.S. Pat. No. 3,695,096 nor U.S. Pat. No. 4,165,637 is effective for this purpose.
On the other hand, U.S. Pat. No. 4,199,978 discloses forming a hole along the length of an elongated member and to arrange a transducer, which has attachment parts at both ends thereof and a measuring part at a location between both ends thereof, in the hole in such a way that the attachment parts are symmetrical relative to the point of action of each load, to thereby measure the angle between two tangents to an elastic deformation line to be developed in the elongated member by the load. It is indispensable in the above structure to arrange the attachment parts symmetrically relative to the point of action of each load. Accordingly, the installation of the transducer must be conducted with special care. It is thus accompanied by a potential danger that some detection errors may arise due to possible offset of the installation position. In general machine parts or the like, the points of action of loads may often change. This also leads to detection errors. Consequently, certain limitations are imposed on the apparatus or method of the U.S. patent, with respect to applicable machine parts.